This invention relates generally to novel ultra high density information recording or storage and readout or playback system utilizing recording densities in the atomic scale range.
In the various arts known for recording information and readout or playback of recorded information, artisans continue to pursue greater recording densities. The most popular example of such endeavors of increasing storage densities is the present development of magnetic recording media having perpendicular magnetic anisotropy to increase magnetic recording densities as compared to that available in in-plane magnetic anisotropy. More recently, optical type and magneto-optic type of recording systems, i.e., systems of the type depending upon the employment of a beam of light for reading or writing and limited in spot size by the wavelength of light used, have been developed and some have entered the market place with recording densities of several magnitudes greater than that available for magnetic recording media.
Recently, there has been developed a high resolution microscopy technique known as scanning tunneling microscopy (STM). STM permits the resolving of features on a surface in the range below 10 .ANG. (1 nm), e.g., vertical feature positions as small as 0.1 .ANG. (0.01 nm) and horizontal feature separations down to 6 .ANG. (0.6 nm). A discussion of STM can be found in U.S. Pat. No. 4,343,993 to Binnig et al. and in the articles of G. Binning et al. "Surface Studies by Scanning Tunneling Microscopy", Physical Review Letters; Volume 49, No. 1, pp. 57-61 (July 5, 1982) and G. Binning et al. "7.times.7 Reconstruction On Si (111) Resolved in Real Space", Physical Review Letters, Volume 50, No. 2, pp. 120-123 (January 10, 1983).
As made known from these references, the "tunneling effect" is utilized to determine contour and the topography and, for example, of crystalline surfaces. The effect is based upon the probability that a limited number of elecrons are capable of tunneling through a barrier comprising, for example, a thin blocking layer in a solid body, e.g. a conductor or semiconductor material. A high vacuum also represents such a tunnel barrier. The tunnel effect recognizes that a limited number of electrons in a potential field can tunnel the barrier even at low potential differences. The number of electrons per unit time that penetrate the barrier, e.g., a vacuum gap, is termed tunnel current. The tunnel current established across a vacuum gap established between two solid surfaces, e.g., a pair of spaced electrodes, will vary as the distance between the two is varied. However, the gap must be very small e.g., in the range of 10 .ANG. (1 nm) to 100 .ANG. (10 nm). Thus, if one electrode is a surface whose topography is under investigation and the other electrode is a very fine metal point, probe or stylus, for a fixed potential difference between the electrodes, the tunnel current will vary with the minute irregularities in distance between the probe and the surface being examined as the probe is scanned across its surface.
Rather than measure a varying tunnel current, it is the objective of Binnig et al. to maintain a constant tunnel current as the probe scans the examined surface. This is accomplished by moving the probe tip toward and away from the examined surface to maintain a constant spacing between the probe tip and the surface. This can be done by monitoring the tunnel current and utilizing it as feedback signal to maintain the probe tip continuously at a predetermined spacing from the surface under investigation. A highly accurate piezoeelctric driver, for example, may move the probe toward or away from the examined surface as the tunnel current respectively decreases or increases. The probe is scanned in the X and Y directions above the examined sample by X and Y piezo drivers as the Z or vertical piezo driver constantly changes the probe vertical position in accordance with varations in the sample contour by means of a closed loop feedback control system. The varying driving voltage of the Z piezo driver represents aa electronic image of the examined surface contour assuming that the work function is constant.
Taking into account the very small Angstrom range detecting capabilities of STM, this capability may be harnessed to provide a recording and playback storage technique having an ultra high recording density several orders of magnitude greater than the best presently known storage densities obtainable with optical disk recording systems. In these optical disk systems, the recording and readout is limited by the size of the optical spot, which in turn is determined by the wavelength of light used. For example, the spot size may be of the order of one micron. By contrast, using the concept of the tunnel current effect, a recorded spot or representation could be created in the 5 .ANG. (0.5 nm) to 10 .ANG. (1 nm) range.
The problem, however, is how to utilize the tunnel current effect in a practical manner to provide such an ultra high density recording and readout system.